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I was told by A/C guy I have a short somewhere to the high pressure switch. something is not kicking on the compressor. another mechanic said I need a new computer. what do you think

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Go with the ac guy
there are systems that prevent the ac compressor working
high pressure switch which cuts the ac clutch if the head pressure goes too high
low pressure switch which cuts power to the ac compressor if there
is no gas in the system
climate control systems which cut power when the ac is not required
fuses to the ac system
get a better explanation or seek advice from other ac mechanics

Posted on May 02, 2015


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A/C Compressor won't turn on Can jump compressor from battery and compressor comes on & cools. Hi pressure switch checks out ok and 12 volts going to hi pressure switch no voltage to compressor

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The ac compressor kicks on and off. We have had a professional look at it, but they couldn't find a problem. However, the compressor doesn't run long enough to get the air cold. Is this an...

Could be either. Usually if the compressor is cycling than its usually low on refrigerant. Could be pressure sensor. Could be cooling fan. I would have a different tech look at it. It is hard to diagnose what is going on without knowing pressures of the high and low side from a gage hooked up to the system.

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My 2004 chevy 1500 v8 air compressor will not kick in. I have jumped the low pressure switch and the compressor will not come on. i have changed the temperature controller and still won't kick in. i...

If the compressor kick's in when you jumper the relay socket there is no problem with the electrical to the compressor an the compressor isn't locked up . You have a control side problem , the PCM - engine computer is suppose to energize the relay if no other problems are present . The proper way to check this would be to hook up a scan tool an check pressure input's to the PCM an is the PCM getting a request from the HVAC control panel to turn on the A/C compressor . AutoZone is wrong as usual , There can be codes without the check engine light being on . o'reilly's is wrong as well DTC P0530 (LU3, LR4, LM7, LQ4 and L18)
The powertrain control module (PCM) or engine control module (ECM) monitors the high side refrigerant pressure via a A/C refrigerant pressure sensor. When the pressure is high the signal voltage is high. When the pressure is low the signal voltage is low. When pressure is too high the PCM will not allow the A/C compressor clutch to engage.

Action Taken When the DTC Sets
The malfunction indicator lamp (MIL) will not illuminate.
Diagnostic Aids
The air conditioning (A/C) compressor clutch will not engage under the following conditions:
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• The A/C low side line pressure is under 138-172 kPa (20-25 psi).
• Throttle angle is at 100 percent.
• Engine speed is more than 5500 RPM.
• Engine coolant temperature (ECT) is more than 121°C (250°F).
• Ambient air temperature is less than 5°C (40°F).
• Engine is idling at a low unstable RPM
Compressor clutch engagement cannot be used to determine status of the low pressure switch. The low pressure switch is one of several inputs to the HVAC control module for A/C request authorization. A/C request is one of several inputs to the powertrain control module (PCM) that control A/C compressor clutch engagement.
To accurately determine what pressure the A/C low pressure switch opens and closes at use Kent Moore GE-47742 which will allow the technician to measure the switch point pressure at the switch.
Using a scan tool, monitor the "low pressure switch" status while monitoring the "low side" pressure at the switch to determine the switch points of the low pressure switch.
The low pressure switch "connector seal" must be removed before plugging it into the switch for testing. The "plunger effect" of plugging the connector with seal into the switch induces a pressure on the back side of the switch, this pressure will skew the opening/closing characteristics of the switch 5-10 psi until the pressure bleeds off. The time required for the connection induced pressure to bleed off can take 20 minutes or longer

Do you have the proper diagnostic tools ? Factory scan tool , A/C gauges ? You best bet , take your vehicle to a qualified repair shop !!

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1 Answer

After replacing a high pressure switch on my 98 gmc sierra a/c

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My airconditioner is blowing hot air. The mechanic said that the compressor is not broken. He needed to find the lower side pressure valve to kick the compressor on so it would stay on. Where is the valve?

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